The present invention relates to a method of preparing soluble glycol methacrylate or glycol acrylate polymers and coplymers.
This application is a companion to our copending application Ser. No. 297,390 filed on even date hereof, also based on Czech. Application No. 7234/71, wherein there has been disclosed a method of forming polymer products and copolymer products of a type similar to that of the present invention but containing both soluble and non-soluble portions. Reference to the copending application is made as if more fully set forth for such definitions of terms, materials, monomers, cross-linking agents, diluents, solvents, etc. as may be required.
Polymers of the above type are usually prepared by free-radical solution polymerization in which a solvent for both the starting components and the resulting polymer product is used. (See U.S. Pat. No. 3,578,946). N,N-dimethyl formamide, dimethyl sulfoxide, pyridine, lower glycols, lower glycol monoethers, lower aliphatic alcohols and acids, etc., are examples of the suitable solvents. In the case of glycol monomethacrylate or glycol monacryate copolymers having other monoolefinic monomers, such as methyl methacrylate, styrene, vinyl acetate, acrylonitrile, the solvents may differ although they always are a solvent which unlimitedly dissolves the resulting polymer product.
The method of production mentioned above, has an undeniable advantage in enabling the preparation of completely soluble polymer products, even in the presence of large amounts of cross-linking comonomer, e.g. ethylene glycol-bis-methacrylate, and even if a high conversion is achieved. However, a disadvantage lies in the fact that it is rather difficult to isolate the polymer product from the solution, after the polymerization process is finished. Although it is sometimes possible, for some technical applications, to use the polymer solution directly after completing the polymerization process and without isolation of the polymer product from this solution, for a majority of other conventional applications however it is necessary to clean out the polymer product, precipitating it with a suitable precipitating agent, or evne isolate it in the form of a dry fine powder.
Precipitation of the polymer product is generally achieved by a large excess of the precipitating agent (about a tenfold amount to the volume of the polymer solution), and it is true, under vigorous stirring. Large volumes of the precipitating agents, often very volatile and violently inflammable organic liquids (e.g. diethyl ether) require great space, and in addition they increase production costs. Additional working operations are normally required such as separation of the polymer precipitated from the precipitating bath, drying the polymer and subsequently disintegrating it into finer particles.
Drying of the polymer product may be performed under precise and gentle conditions only, best at room temperature, and with the use of vacuum. Otherwise, at a higher temperature (above 80.degree. C), for example, the polymer particles may agglomerate into bigger bits, or pieces and may even be converted into an undesirable insoluble form, as a result of the additional cross-linking. In some cases it is possible to use pure water as the precipitating agent for the polymer product. This may appear at first as very advantageous in comparision with more expensive, and inflammable organic liquid-precipitating agents. In reality, however, great difficulties occur in keeping large quantities of water in the swollen polymer particles, and it must be pointed out, that subsequent removal of this water is not easy. In the latter system, in the beginning of the drying process alone, a thin glassy layer is formed on the surface of the polymer particles, which inhibits further water evaporation from within the middle of these particles, and thus hinders the drying process and causes it to go on at an unreasonable rate.
The disintegration of larger particles, most often performed by milling, is also a rather expensive operation, and in addition, the resultant product is often contaminated with small foreign bodies, e.g. dust, metal, and the like.
It is the object of the present invention to provide a method of producing a soluble polymer which overcomes the difficulties of the prior art.
It is another object of the present invention to provide a method for producing completely soluble glycol methacrylate or glycol acrylate polymers or copolymers wherein a starting monomer or the monomer mixture having a minor amount of a cross-linking agent such as a corresponding glycol multifold-methacrylate or glycol multifold-acrylate monomer, which represent a conventional, but often undesirable admixture of corresponding monoesters, is combined with a diluent and polymerized under free-radical conditions in such a way, that the concentration of the multifold esters in the whole polymerization mixture including diluent decreases under the critical limit of 0.02 percent by volume. The diluent comprises a solvent unlimitedly dissolving all starting components but being neither a solvent for, nor an agent substantially swelling the resulting polymer product.
The proper polymerization process is performed by the precipitating technique, well-known itself, which in comparison with a free-radical solution polymerization has many advantages among which is an increased speed of polymerization in the entire range of conversion of the monomers to the polymer product; obtaining of fine discrete polymer particles of about 1 .mu. size capable of isolation from the polymerization mixture, by filtration or centrifugation or the like; and a product which is easy to dry into a fine polymer powder at relatively low production cost. (See FIG. 1).